1. Field of the Invention
The present invention relates to an actuator used in an optical scanner, an angular velocity sensor, or the like.
2. Description of the Related Art
U.S. Pat. No. 6,813,049 B2 discloses an electromagnetically driven actuator. This actuator, which is fabricated by processing an SOI substrate, comprises a flat movable plate, a pair of elastic members of an active layer supporting the flat movable plate to allow it to rock, and a frame-shaped support supporting the elastic members. The movable plate has a flat coil on its upper surface near the peripheral portion and a reflection mirror on its lower surface. The support is provided with permanent magnets. The two ends of the flat coil are electrically connected to electrode pads arranged on the support through wirings extending on the elastic members.
FIG. 7 shows the sectional structure of the elastic member in the actuator. As shown in FIG. 7, an elastic member 210 comprises a silicon active layer 211, an insulating film 212 of a silicon oxide film formed on the silicon active layer 211, a wiring layer 213 of aluminum formed on the insulating film 212, and a protective film 214 of a silicon oxide film covering the wiring layer 213.
In the conventional actuator, due to the torsional motion of the elastic member 210, a stress repeatedly acts on the wiring layer 213 and the protective film 214. As a result, small voids are generated in the wiring layer 213 of a metal such as aluminum, in which atoms tend to move readily. The small voids increase the volume of the wiring layer 213, so that, in addition to the stress of the torsional motion, a stress from the wiring layer 213 also acts on the protective film 214. As far as the stresses remain within the elastic deformation region of the protective film 214 that is on the wiring layer 213 to be in contact with it, no problem occurs. If the stresses exceed the elastic deformation region of the protective film 214, cracks are generated in the protective film 214 undesirably. Accordingly, the stress acting on the wiring layer 213 from the protective film 214 contacting with the wiring layer 213 changes depending on the locations. This causes a difference in mobility of the metal atoms in the wiring layer 213 depending on the locations, and the small voids partially grow to large voids. When the voids reach the surface, cracks are generated in the wiring layer 213 as well. These cracks may change the resistance of the wiring layer 213 to adversely affect rocking of the actuator. Due to the presence of the cracks in the protective film 214, the wiring layer 213 may undesirably communicate with the outer air to decrease the reliability such as humidity resistance of the actuator.